1. Field of the Invention
The present invention relates to a magnetic head for use in a magnetic recording and reproducing system, and more particularly, to such a magnetic head being excellent in magnetic properties, resistance to abrasion, durability, and mass producibility, and to a process for producing the same, and to a magnetic recording and reproducing system using the magnetic head.
2. Description of Related Art
There have been employed ferrites as magnetic core material in conventional magnetic heads. For bonding magnetic cores in the heads, non-magnetic glasses having a thermal expansion coefficient matching with that of ferrites, and a bonding temperature of about 700.degree. C. have been used taking account of requirements on resistance to adverse environment conditions, resistance to abrasion, and strength. Generally, as glasses have a higher bonding temperature, they become lower in thermal expansion coefficient, and higher in resistance to weather, resistance to abrasion, and strength.
Recently, in order to increase greatly a recording density of magnetic recording and reproducing systems such as VTRs and the like, an attempt has been to increase the coercive force of recording mediums and the speed of recording tapes and disks. In connection with this, magnetic alloy films exhibiting a saturation magnetization of not less than 8000 G have been employed as materials for the magnetic cores used in the magnetic heads. Such magnetic heads may have a structure where a pair of magnetic cores each having a magnetic alloy film formed on a substrate are abutted on each other face to face with a non-magnetic spacer being interposed there-between, and bonded with glasses. For this reason, when the magnetic heads are produced, both the thermal expansion coefficient of the bonding glass and of the substrate must match that of the magnetic alloy film. The most attractive alloys for use in the magnetic alloy films at present are Co based amorphous alloys and Sendust alloys (Fe--Si--Al system alloys). Generally, such alloys have a higher thermal expansion coefficient than that of ferrites, for example, the Co based amorphous alloys exhibit about 120.times.10.sup.-7 /.degree.C. and Sendust alloys about 150.times.10.sup.-7 /.degree.C. For this reason, one must employ glasses having almost the same thermal expansion coefficient as that of the magnetic alloy films to bond the magnetic cores. If a difference in thermal expansion is higher, the magnetic alloy film is stressed causing problems that the magnetic properties are impaired and that separation of the film and/or the glasses occurs. Therefore, the bonding glasses for ferrites can not be used for the heads using the magnetic alloy films. Thus, PbO--B.sub.2 O.sub.3 system glasses containing a major component PbO which exhibit a high thermal expansion coefficient has been employed. This type of glass is characterized by having a higher thermal expansion coefficient with a higher content of PbO, and being capable of lowering its bonding temperature to 500.degree. C. or less. For this reason, this type of glass has been particularly useful when the Co based amorphous alloys are used as the magnetic alloy films because bonding can be effected without causing crystallization of the alloys. Comparing with the bonding glasses used for ferrite heads, however, they are very inferior in mechanical properties and resistance to weather causing problems that they are susceptible to cracking, fracture, and erosion with a washing liquid during processing, assembling, or washing the magnetic heads. This has resulted in a poor yield of the magnetic heads in the production thereof. There have been another problems where the glass bondings are susceptible to scratches and abrasion when the recording tapes are allowed to run sliding on the head. These difficulties may cause adhesion of the recording mediums on the magnetic heads and deterioration of the magnetic properties. In order to overcome the difficulties, there have been proposed glass compositions and magnetic heads as described in Japanese Patent KOKAI (Laid-Open) Nos. 62-78128 and 62-88109. These prior inventions employ V.sub.2 O.sub.5 --P.sub.2 O.sub.5 --Sb.sub.2 O.sub.3 system glasses having a thermal expansion coefficient of 100.times.10.sup.-7 /.degree.C. When Sendust alloys are used as magnetic alloy films, bonding of the magnetic cores must be performed at a heat-resistance temperature, under which the alloys are kept intact, less than about 650.degree. C. The magnetic properties of Sendust alloys are extremely deteriorated at temperatures higher than its heat-resistance temperature. Such difficulties can be avoided by PbO--B.sub.2 O.sub.3 system glasses which permit glass bonding to be formed at lower temperatures. Since Sendust alloys have a very high thermal expansion coefficient of about 150.times.10.sup.-7 /.degree.C., however, even with PbO--B.sub.2 O.sub.3 system glasses any matching in thermal expansion coefficient between them could not be achieved. This type of glasses has at most a thermal expansion coefficient of about 130.times.10.sup.-7 /.degree.C. Another problem of this type of glasses is that as the thermal expansion coefficient becomes higher, it has a lower bonding temperature, but becomes poor in mechanical properties and resistance to weather. In order to overcome these difficulties, such adhesives as disclosed in Japanese Patent KOKAI (Laid-Open) No. 61-158860 have been proposed. This prior invention intended to match the thermal expansion coefficient of the PbO--B.sub.2 O.sub.3 --SiO.sub.2 system glasses containing a major component PbO to those of Sendust alloys by incorporating Na.sub.2 O and/or CaO into the glasses.
The prior techniques as described above do not employ suitable bonding glasses for the high performance magnetic heads using the magnetic alloy films, and therefore, could not satisfy requirements on magnetic properties, resistance to abrasion, durability, and mass-producibility. Inventions disclosed in Japanese Patent KOKAI (Laid-Open) Nos. 62-78128 and 62-88109 have caused problems that deterioration of the magnetic properties of the magnetic alloy films, and separation of the magnetic alloy films and the bonding glasses occur due to inferior matching in the thermal expansion coefficient between the magnetic alloy films and that of the bonding glasses. The invention disclosed in Japanese Patent KOKAI (Laid-Open) No. 61-158860 again have caused problems where the bonding glasses are very inferior in mechanical properties and resistance to weather so that they are susceptible to cracking, fracture, and erosion with a washing liquid during processing, assembling, or washing the magnetic heads. Moreover, the invention has another problem where the glass bondings are susceptible to scratches and abrasion when the recording tapes are allowed to run sliding on the magnetic heads.